The present invention relates to a control system for adjusting displacement of a variable displacement compressor of a refrigerant circuit (a refrigeration cycle) in an air conditioner and is configured to optionally vary the displacement, while refrigerant gas is compressed by rotation of a drive shaft of the compressor.
As disclosed in page 7 to 11 and FIG. 3 of Unexamined Japanese Patent Publication No. 2001-173556, a control system of the above type includes an external control valve having an electromagnetic actuator in a pressure sensing valve. Namely, the external control valve includes a valve body, a pressure sensing member and the electromagnetic actuator. The valve body optionally adjusts the opening degree of a supply passage that interconnects a discharge chamber of a variable displacement swash plate type compressor (hereinafter, the compressor) and a crank chamber, which is an accommodating chamber for accommodating a swash plate of the compressor. The pressure sensing member mechanically detects pressure difference between two pressure monitoring points located in a discharge pressure region in a refrigerant circuit. The pressure difference between the above two points reflects the flow rate of refrigerant in the refrigerant circuit. The pressure sensing member moves the valve body in such a manner that the displacement of the compressor is varied to cancel the variation n of the pressure difference between the above two points, that is, the variation of the flow rate of refrigerant.
The above electromagnetic actuator varies electromagnetic urging force (particularly, urging force that resists against urging force applied to the valve body by the pressure sensing member in a direction to open the valve) applied to the valve body in a direction to close the valve by electric power externally supplied so that a set pressure difference between the two pressure monitoring points is optionally varied. Incidentally, the set pressure difference is a reference value for positioning the valve body by the pressure sensing member. Namely, for example, as the electric power externally supplied to the electromagnetic actuator increases, the electromagnetic actuator strengthens the electromagnetic urging force applied to the valve body and increases the set pressure difference. On the contrary, as the electric power externally supplied to the electromagnetic actuator decreases, the electromagnetic actuator weakens the electromagnetic urging force applied to the valve body and decreases the set pressure difference.
The flow rate of refrigerant in the refrigerant circuit positively correlates with the displacement of the compressor and the rotational speed of the vehicle engine for driving the compressor. Generally, the maximum value of the set pressure difference, that is, the maximum value of the electromagnetic urging force applied to the valve body by the electromagnetic actuator, is predetermined at a flow rate of refrigerant that is optionally performed in a state when the displacement of the compressor is maximum and the engine is rotated in a range of regular rotational speed. Accordingly, even if the displacement of the compressor is maximum, the flow rate of refrigerant corresponding to the maximum set pressure difference is impossibly performed in a state when the engine is rotated in a range of relatively low rotational speed, which is close to an idling of the engine.
An unwanted feature is that in a prior art since the rotational speed of the engine is not reflected to calculate the set pressure difference (the magnitude of electric power supplied to the electromagnetic actuator), the impossibly performed flow rate of refrigerant between the two pressure monitoring points is possibly ordered to the electromagnetic actuator in a state when the engine is rotated at a range of relatively low rotational speed. Accordingly, for example, when cooling is required, the set pressure difference ordered to the electromagnetic actuator largely deviates from the optionally performed pressure difference between the two pressure monitoring points at the moment in such a manner that the set pressure difference is greater than the pressure difference between the two pressure monitoring points.
Even if the rotational speed of th compressor rapidly increases due to th rapid acceleration of the vehicle and tends to increase the flow rat of refrigerant in the refrigerant circuit in the above state, the valve body cannot leave from a fully-closed state until the flow rate of refrigerant increases to correspond to the set pressure difference ordered to the electromagnetic actuator. Accordingly, it takes a relatively long time to initiate to leave from the maximum displacement of the compressor after the engine commences rapid increasing in rotational speed. As a result, discharge pressure of the compressor excessively increases so that a problem, such as a trouble with the compressor or with a conduit of the refrigerant circuit, has occurred.
Not only the above problem occurs in the control valve that has the pressure sensing member to sense the pressure difference between the two monitoring points in the refrigerant circuit, but also a similar problem occurs in a control valve that has a pressure sensing member to move by detecting at least one kind of pressure in the refrigerant circuit. Namely, for example, even if a control valve optionally varies set suction pressure in such a manner that the pressure sensing member senses pressure in a suction pressure region in the refrigerant circuit, the set suction pressure ordered to the electromagnetic actuator is possibly set to an excessively low value that is impossibly performed in the state of relatively low rotational speed of the engine at the moment when cooling is required.
Incidentally, a relief valve may be arranged in a discharge pressure region or a means may be employed for decreasing the displacement of the compressor by detecting acceleration of the vehicle through an acceleration pedal and the like. However, when the relief valve is applied, the relief valve needs be exclusive so that the number of components increases. When the means for decreasing the displacement of the compressor is applied, in a state when discharge pressure just before rapid acceleration of the vehicle is relatively high, an external control after detecting the rapid acceleration is so late that the discharge pressure excessively increases. Therefore, there is a need for a control system that immediately decreases the displacement of a compressor from the maximum and prevents an excessive increase in discharge pressure when rotational speed of the compressor rapidly increases.